1. Field of the Invention (Technical Field)
The present invention relates to methods and apparatuses for summing coherent laser beams.
2. Description of Related Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
In order to effectively sum coherent laser beams to produce a high power single beam, with their combined power and intensity, in a highly coherent output to maximize spatial brightness, it is necessary that the beams be of the same wavelength, exhibit the same intensity and phase profiles (“mode”) and overlap and coexist in a path that is colinear and coincident. Furthermore, there must be a phase relation between the beams. To achieve these conditions it is typically necessary to use a separate and external laser to seed the lasers producing the beams being combined, as shown in FIG. 1. This complex and expensive traditional method has at least the following disadvantages: (1) Combined output beam quality fluctuates with beam splitter vibration jitter; (2) Laser cavity tuning response time is limited by PZT (piezo-electric transducer) response time; and (3) Co-linearity and overlap of output beams is compromised by hardware limitations. The present invention describes a method and apparatus for seeding the lasers without using a separate laser and for combining the beams.
Prior phase-locking of an array of lasers has involved either using an external laser to seed the lasers producing the beams being combined, or a method of measuring the quality and/or efficiency of the beam combining and using this information in a feedback servo to adjust the phase of each laser output beam (usually by length tuning the laser cavity mirror separation). A more recent attempted solution has been provided by H. Bruesselbach et al., “Self-organized coherence in fiber laser arrays”, OPTICS LETTERS, 30(11), 1339-1341 (Jun. 1, 2005), which employs a one-to-five coupler from Gould Electronics to cause in-phase states to occur. The disadvantages of this approach include: (1) The steady-state, self-organized coherence and laser output mode solution described is complex (possibly involving new applications and advancements of existing theory) and thus it will be difficult to analyze and predict/optimize the performance a particular array of lasers. (2) Unlike the lasers in FIG. 1 in which the lasers to be combined all march in parallel to the beat of the external seed laser and local oscillator “drum” (although the lasers are “trimmed” individually by their heterodyne detector feed back servos), the lasers combined using a self-organized coherence technique, by its very nature involves the in-series rather than in-parallel conforming of phase. “In-series” can often be accompanied by accumulation of undesirables including phase instability which can translate into combined output beam intensity and phase instability. (3) In some cases and particular setups of an array of lasers to be combined, the extraction of the phase information from each and every laser and the communication of this information to each and every laser can burden or “drain” the available combined laser power and reduce the overall combined laser efficiency. (4) In some cases and particular setups of an array of lasers to be combined, the addition of the components required to accomplish the extraction and communication of the phase information from each and every laser can be expensive or impractical.